CN112840045A - Hot-rolled steel sheet and method for producing same - Google Patents

Hot-rolled steel sheet and method for producing same Download PDF

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Publication number
CN112840045A
CN112840045A CN201980067649.6A CN201980067649A CN112840045A CN 112840045 A CN112840045 A CN 112840045A CN 201980067649 A CN201980067649 A CN 201980067649A CN 112840045 A CN112840045 A CN 112840045A
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steel sheet
hot
rolled steel
less
average
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CN112840045B (en
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横井龙雄
首藤洋志
林田辉树
安藤洵
榊原睦海
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Abstract

The hot-rolled steel sheet has a predetermined chemical composition, and when the thickness is t, the metal structure at a position t/4 away from the surface contains 77.0-97.0% of bainite or tempered martensite, 0-5.0% of ferrite, 0-5.0% of pearlite, 3.0% or more of retained austenite, and 0-10.0% of martensite in terms of area fraction; in the microstructure, the average grain size of the iron-based carbides excluding the retained austenite is 7.0 μm or less, and the average number density of the iron-based carbides having a diameter of 20nm or more is 1.0X 106Per mm2The above; tensile strength is over 980 MPa; of the above surfacesThe average Ni concentration is 7.0% or more.

Description

Hot-rolled steel sheet and method for producing same
Technical Field
The present invention relates to a hot-rolled steel sheet and a method for producing the same.
The present application claims priority based on Japanese application laid-open No. 2018-197936 at 10/19/2018, and the contents thereof are incorporated herein by reference.
Background
In recent years, carbon dioxide (CO) from automobiles has been suppressed2) The emission amount of (2) has been reduced by using high-strength steel sheets. In addition, in order to ensure safety of passengers, high-strength steel sheets are used in large quantities in automobile bodies in addition to mild steel sheets.
Furthermore, recently, NO is caused by fuel consumption restrictionXSuch as further tightening of environmental restrictions, an increase in plug-in hybrid vehicles and electric vehicles is expected. In these next-generation automobiles, a large-capacity battery needs to be mounted, and further reduction in vehicle body weight is required. In automobile manufacturers, technology development for reducing the weight of vehicle bodies for the purpose of reducing fuel consumption has been actively conducted. However, since emphasis is placed on improvement of the collision resistance property in order to ensure safety of passengers, it is not easy to reduce the weight of the vehicle body.
In order to further reduce the weight of the vehicle body, replacement of the steel sheet with a light-weight material such as aluminum alloy, resin, CFRP, or further strengthening of the steel sheet may be an option, but from the viewpoint of material cost and processing cost, it is realistic to use an ultra-high strength steel sheet for mass production of mass-produced mass-market vehicles other than high-grade vehicles.
In order to achieve both weight reduction of the vehicle body and collision resistance, it has been studied to reduce the thickness of the member using a high-strength steel sheet. Therefore, a steel sheet having both high strength and excellent formability is strongly desired, and several techniques have been proposed in order to meet these requirements. Among them, a steel sheet containing retained austenite shows excellent ductility due to transformation induced plasticity (TRIP), and thus many studies have been made so far.
For example, patent document 1 discloses a high-strength steel sheet for automobiles, which is excellent in collision safety and formability, and in which retained austenite having an average crystal grain size of 5 μm or less is dispersed in ferrite having an average crystal grain size of 10 μm or less. In a steel sheet containing retained austenite in the microstructure, austenite undergoes martensitic transformation during working, and exhibits a large elongation by inducing plasticity through transformation, but the hole expansibility is impaired by the formation of hard martensite. Patent document 1 discloses that not only ductility but also hole expansibility is improved by making ferrite and retained austenite fine.
Patent document 2 discloses a high-strength steel sheet having a tensile strength of 980MPa or more, which is excellent in elongation and stretch flangeability, wherein a second phase containing retained austenite and/or martensite is finely dispersed in grains.
Patent documents 3 and 4 disclose a high-tension hot-rolled steel sheet having excellent ductility and stretch flangeability, and a method for producing the same. Patent document 3 discloses a method for producing a high-strength hot-rolled steel sheet having excellent ductility and stretch-flange formability, in which the steel sheet is cooled to a temperature range of 720 ℃ or lower within 1 second after completion of hot rolling, retained at a temperature range of more than 500 ℃ and 720 ℃ or lower for a retention time of 1 to 20 seconds, and then coiled in a temperature range of 350 to 500 ℃. Patent document 4 discloses a high-strength hot-rolled steel sheet having excellent ductility and stretch flangeability, which mainly comprises bainite, contains an appropriate amount of polygonal ferrite and retained austenite, and has an average grain size of 15 μm or less of crystal grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in a steel structure other than the retained austenite.
On the other hand, recently, LCA (Life Cycle Assessment) has become of interest, and attention is paid not only to traveling of an automobile but also to environmental load at the time of manufacture.
For example, in coating of automobile parts, zinc phosphate treatment, which is one of chemical conversion treatments, has been carried out as a base treatment. The zinc phosphate treatment is low in cost, and excellent in coating adhesion and corrosion resistance. However, the zinc phosphate treatment liquid contains phosphoric acid as a main component and metal components such as zinc salt, nickel salt, and manganese salt. Therefore, there is a concern about environmental load due to phosphorus and metals in the waste liquid discarded after use. In addition, a large amount of sludge mainly composed of iron phosphate precipitated in the chemical conversion treatment tank becomes a large environmental load as industrial waste.
Therefore, in recent years, zirconium-based chemical conversion treatment liquids have been used as chemical conversion treatment liquids that can reduce environmental load. The zirconium-based chemical conversion treatment liquid does not contain a phosphate, and it is not necessary to add a metal salt. Therefore, the sludge generation amount is extremely small. For example, patent documents 5 and 6 describe techniques for forming a chemical conversion coating on a metal surface using a zirconium chemical conversion treatment liquid.
Documents of the prior art
Patent document
Patent document 1: japanese laid-open patent publication No. 11-61326
Patent document 2: japanese patent laid-open publication No. 2005-179703
Patent document 3: japanese patent laid-open publication No. 2012-251200
Patent document 4: japanese patent laid-open publication No. 2015-124410
Patent document 5: japanese patent laid-open publication No. 2004-218074
Patent document 6: japanese laid-open patent publication No. 2008-202149
Disclosure of Invention
Problems to be solved by the invention
Even if a zirconium-based chemical conversion treatment liquid is used, corrosion resistance and coating film adhesion comparable to those of zinc phosphate treatment can be obtained even for conventional high-strength steel sheets of strength grade of 780 MPa. However, in the case of an ultrahigh-strength steel sheet having a tensile strength of 980MPa or more, the amount of alloying elements contained is large, so that the adhesion of zirconium-based chemical conversion crystals to the surface of the steel sheet is insufficient, and good corrosion resistance and coating film adhesion are not obtained.
Further, with respect to ultrahigh-strength steel sheets excellent in collision resistance characteristics, such as the steel sheets disclosed in patent documents 1 to 4, no method has been proposed for sufficiently improving the coating adhesion when a zirconium-based chemical conversion treatment liquid is used.
The present invention has been made in view of the above problems, and an object thereof is to provide a hot-rolled steel sheet which has a tensile strength of 980MPa or more, a high press formability (ductility and stretch flangeability), and a good toughness, and which has a chemical conversion treatment property and coating adhesion equal to or higher than those of a zinc phosphate chemical conversion treatment liquid even when a zirconium-based chemical conversion treatment liquid is used, and a production method capable of stably producing the hot-rolled steel sheet.
Means for solving the problems
The present inventors have conducted intensive studies to solve the above problems and have obtained the following findings.
The present invention has been made based on these findings, and the gist thereof is as follows.
(1) A hot-rolled steel sheet according to an aspect of the present invention has a chemical composition expressed by an average value in a sheet thickness direction as a whole, and includes, in mass%: c: 0.100 to 0.250%, Si: 0.05 to 3.00%, Mn: 1.00-4.00%, Al: 0.001 to 2.000%, Ni: 0.02 to 2.00%, Nb: 0-0.300%, Ti: 0-0.300%, Cu: 0-2.00%, Mo: 0-1.000%, V: 0-0.500%, Cr: 0-2.00%, Mg: 0-0.0200%, Ca: 0-0.0200%, REM: 0-0.1000%, B: 0 to 0.0100%, Bi: 0 to 0.020%, 1 or more than 2 of Zr, Co, Zn and W: 0 to 1.000% in total, Sn: 0-0.050%, P: 0.100% or less, S: 0.0300% or less, O: 0.0100% or less, N: 0.1000% or less, the remainder comprising Fe and impurities, and satisfying the following formula (i); when the thickness is t, the metal structure at a position t/4 away from the surface contains 77.0 to 97.0% of bainite or tempered martensite and 0 to 5.0% of iron in terms of area fractionFerrite, 0 to 5.0% of pearlite, more than 3.0% of retained austenite and 0 to 10.0% of martensite; in the metal structure, the average grain size of the iron-based carbides excluding the retained austenite is 7.0 μm or less, and the average number density of the iron-based carbides having a diameter of 20nm or more is 1.0X 106Per mm2The above; tensile strength is over 980 MPa; the average Ni concentration in the surface is 7.0% or more.
Si + Al is more than or equal to 0.05% and less than or equal to 3.00% of formula (i)
The elements represented by the above formula (i) are mass% of the elements contained in the above hot-rolled steel sheet.
(2) The hot-rolled steel sheet according to the above (1), wherein the chemical composition may further contain, in mass%: 0.02-0.05%.
(3) The hot-rolled steel sheet according to the above (1) or (2), wherein an internal oxidized layer may be present in the hot-rolled steel sheet, and an average depth of the internal oxidized layer is 5.0 μm or more and 20.0 μm or less from the surface of the hot-rolled steel sheet.
(4) The hot-rolled steel sheet according to any one of the above (1) to (3), wherein a standard deviation of an arithmetic average roughness Ra of the surface of the hot-rolled steel sheet may be 10.0 μm or more and 50.0 μm or less.
(5) The hot-rolled steel sheet according to any one of the above (1) to (4), wherein the chemical composition may contain, in mass%, V: 0.005-0.500%, Ti: 0.005-0.300% of 1 or 2.
(6) The hot-rolled steel sheet according to any one of the above (1) to (5), wherein the chemical composition may contain, in mass%: 0.005-0.300%, Cu: 0.01% -2.00%, Mo: 0.01% -1.000%, B: 0.0001-0.0100%, Cr: 0.01% to 2.00% of 1 or 2 or more.
(7) The hot-rolled steel sheet according to any one of the above (1) to (6), wherein the chemical composition may contain, in mass%, Mg: 0.0005 to 0.0200%, Ca: 0.0005 to 0.0200%, REM: 0.0005-0.1000% of 1 or more than 2.
(8) A method for manufacturing a hot-rolled steel sheet according to another aspect of the present invention includes the steps of: a heating step of heating a billet having the chemical composition described in (1) above to 1150 ℃ or higher in a heating furnace having at least a preheating zone, a heating zone, and a soaking zone and provided with a regenerative burner; a hot rolling step of hot rolling the heated slab so that the finish rolling temperature is T2 ℃ or higher obtained by the following formula (ii) and the cumulative reduction in the temperature range of 850 to 1100 ℃ is 90% or higher to obtain a hot-rolled steel sheet; a primary cooling step of starting cooling within 1.5 seconds after the hot rolling step and cooling the hot-rolled steel sheet to a temperature T3 ℃ or lower represented by the following formula (iii) at an average cooling rate of 50 ℃/sec or higher; a secondary cooling step of cooling the primary cooling step from the cooling stop temperature to a coiling temperature of (T4-100) DEG C to (T4+50) DEG C at an average cooling rate of 10 ℃/sec or more while setting a temperature represented by the following formula (iv) to T4 ℃; and a winding step of winding at the winding temperature, wherein in the heating step, the air ratio in the preheating zone is set to 1.1 to 1.9.
T2(℃)＝868-396×[C]-68.1×[Mn]+24.6×[Si]-36.1×[Ni]-24.8×[Cr]-20.7×[Cu]+250×[Al] (ii)
T3(℃)＝770-270×[C]-90×[Mn]-37×[Ni]-70×[Cr]-83×[Mo] (iii)
T4 (C) (. degree.c.) -591-.
(9) According to the method for producing a hot-rolled steel sheet described in the above (8), in the heating step, the air ratio in the heating zone may be set to 0.9 to 1.3.
(10) According to the method for producing a hot-rolled steel sheet described in the above (8) or (9), in the heating step, the air ratio in the soaking zone may be set to 0.9 to 1.9.
(11) According to the production method of a hot rolled steel sheet described in the above (9) or (10), the air ratio in the preheating zone may be larger than the air ratio in the heating zone.
(12) The method for producing a hot-rolled steel sheet according to any one of the above (8) to (10), wherein a pickling step of pickling the hot-rolled steel sheet after the coiling step using a 1 to 10 mass% hydrochloric acid solution at a temperature of 20 to 95 ℃ for a pickling time of 30 seconds or longer and less than 60 seconds is further provided.
Effects of the invention
According to the aspect of the present invention, a hot-rolled steel sheet having a tensile strength of 980MPa or more, high press formability (ductility and stretch flangeability), and good toughness can be obtained, which has chemical conversion treatability and coating adhesion equal to or higher than those of a zinc phosphate chemical conversion treatment liquid even when a zirconium-based chemical conversion treatment liquid is used. The steel sheet of the present invention has excellent corrosion resistance after coating because of excellent chemical conversion treatability and coating film adhesion. Further, the composition is excellent in ductility and stretch flangeability. Therefore, the steel sheet of the present invention is suitable for automobile parts requiring high strength, formability, and corrosion resistance after coating.
Drawings
Fig. 1 shows an example of EPMA measurement results of the surfaces of the hot-rolled steel sheet and the relatively hot-rolled steel sheet according to the present embodiment. (measurement conditions: acceleration voltage: 15kV, irradiation Current: 6X 10-8A. Irradiation time: 30ms, beam diameter: 1 μm)
Fig. 2 is a diagram showing a mechanism in which Ni concentrated on the surface becomes a precipitation nucleus of a zirconium-based chemical conversion crystal.
Fig. 3 is a diagram showing a mechanism of change in the roughness of the surface of the hot-rolled steel sheet.
Detailed Description
The present inventors have conducted extensive studies on conditions under which an ultrahigh-strength steel sheet having a tensile strength of 980MPa or more and sufficient ductility and stretch flangeability can be stably obtained by chemical conversion treatment using a zirconium-based chemical conversion treatment liquid, and thus excellent chemical conversion treatability and coating film adhesion can be stably obtained. The results of the study know: the oxide on the surface layer of the steel sheet greatly affects the chemical conversion treatability and the coating adhesion. Specifically, the following is described.
The steel sheet is usually pickled before being subjected to chemical conversion treatment. However, it is known that: even if the ordinary pickling is performed, oxides of Si, Al, and the like are formed on the surface of the ultrahigh-strength steel sheet, which deteriorates zirconium-based chemical conversion treatability and coating film adhesion. The inventor further researches and discovers that: in order to improve the chemical conversion treatability and the coating adhesion, it is effective to suppress the formation of oxides such as Si and Al and form a Ni-enriched layer on the surface layer of the steel sheet as precipitation nuclei of zirconium-based chemical conversion crystals.
In addition, the present inventors have found that: when mass production is assumed at low cost in a general process for producing a hot-rolled steel sheet, a Ni-concentrated layer can be formed on the surface layer of the steel sheet after pickling (before chemical conversion treatment) by limiting the content of a trace amount of Ni and the heating conditions in the heating step before hot rolling.
The hot-rolled steel sheet according to the present embodiment will be described in detail below.
[ composition of Steel sheet ]
First, the reason for limiting the chemical composition of the hot-rolled steel sheet according to the present embodiment will be described. Unless otherwise specified,% of the content of the component represents mass%.
In addition, the expression of the element name used in each formula in the present specification indicates that 0 is substituted in the case where the content (mass%) of the element in the steel sheet is not contained.
C：0.100～0.250％
C has an action of promoting the formation of bainite and an action of stabilizing the residual austenite. When the C content is less than 0.100%, it becomes difficult to obtain a desired bainite area fraction and a desired retained austenite area fraction. Therefore, the C content is set to 0.100% or more. The C content is preferably 0.120% or more or 0.150% or more.
On the other hand, if the C content exceeds 0.250%, pearlite is preferentially formed and the formation of bainite and retained austenite becomes insufficient, and it becomes difficult to obtain a desired area fraction of bainite and area fraction of retained austenite. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.220% or less or 0.200% or less.
Si：0.05～3.00％
Si has a function of delaying the precipitation of cementite. This action can increase the amount of austenite remaining without transformation, that is, the surface area fraction of retained austenite, and can increase the strength of the steel sheet by solid solution strengthening. In addition, Si has a function of strengthening the steel by deoxidation (suppressing generation of defects such as pores in the steel). If the Si content is less than 0.05%, the effects of the above-described effects cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more or 1.00% or more.
On the other hand, if the Si content exceeds 3.00%, the surface properties and chemical conversion treatability, and further ductility and weldability of the steel sheet deteriorate significantly, and the a3 transformation point increases significantly. This makes it difficult to stably perform hot rolling. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less or 2.50% or less.
Mn：1.00～4.00％
Mn has an effect of suppressing ferrite transformation to promote bainite formation. When the Mn content is less than 1.00%, a desired area fraction of bainite cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.50% or more, more preferably 1.80% or more.
On the other hand, if the Mn content exceeds 4.00%, the completion of bainite transformation is delayed, so that the concentration of carbon in austenite is not promoted, the formation of retained austenite becomes insufficient, and it becomes difficult to obtain a desired area fraction of retained austenite. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less or 3.50% or less.
Ni：0.02％～2.00％
Ni is one of important elements in the hot-rolled steel sheet according to the present embodiment. Ni is concentrated mainly in the vicinity of the steel sheet surface near the interface between the steel sheet surface and the scale under specific conditions in the heating step of the hot rolling step. This Ni forms a precipitation nucleus of a zirconium-based chemical conversion coating film when the surface of a steel sheet is subjected to a zirconium-based chemical conversion treatment, and promotes formation of a coating film having no uncoated portion (may be referred to as a bare portion, japanese laid-open text: スケ) and good adhesion. Since this effect is not obtained when the Ni content is less than 0.02%, the Ni content is set to 0.02% or more. The adhesion improving effect can be obtained not only for zirconium-based chemical conversion coatings but also for conventional zinc phosphate chemical conversion coatings. In addition, the adhesion between the base material and the hot dip galvanized layer formed by the hot dip galvanizing treatment and the alloyed galvanized layer formed by the alloying treatment after the plating is also improved.
On the other hand, even if the Ni content exceeds 2.00%, only the effect is saturated and the alloy cost rises. Therefore, the Ni content is set to 2.00% or less. Preferably 0.50% or less, 0.20% or less, or 0.05% or less.
Al：0.001～2.000％
Al has the function of deoxidizing the steel and strengthening the steel sheet, similarly to Si. In addition, Al has an action of promoting the generation of residual austenite by suppressing the precipitation of cementite from austenite. If the Al content is less than 0.001%, the effects of the above-described effects cannot be obtained. Therefore, the Al content is set to 0.001% or more. The Al content is preferably 0.010% or more.
On the other hand, if the Al content exceeds 2.000%, the above effect is saturated, and is not economically preferable. Therefore, the Al content is set to 2.000% or less. The Al content is preferably 1.500% or less or 1.300% or less.
P: less than 0.100%
P is an element generally contained as an impurity, but is also an element having an action of improving strength by solid solution strengthening. P may be positively contained, but P is an element that is easily segregated, and when the content of P exceeds 0.100%, the reduction in formability and toughness due to grain boundary segregation becomes significant. Therefore, the P content is limited to 0.100% or less. The P content is preferably 0.030% or less. The lower limit of the P content is not particularly limited, but is preferably set to 0.001% from the viewpoint of refining cost.
S: less than 0.0300%
S is an element contained as an impurity, and forms sulfide-based inclusions in steel, thereby reducing the formability of the hot-rolled steel sheet. If the S content exceeds 0.0300%, moldability is significantly reduced. Therefore, the S content is limited to 0.0300% or less. The S content is preferably 0.0050% or less. The lower limit of the S content is not particularly limited, but is preferably set to 0.0001% from the viewpoint of refining cost.
N: less than 0.1000%
N is an element contained as an impurity in steel and is an element that degrades the formability of steel sheets. When the N content exceeds 0.1000%, the formability of the steel sheet is remarkably reduced. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, and more preferably 0.0700% or less. The lower limit of the N content is not particularly limited, but when the microstructure is refined by containing 1 or 2 or more types of Ti and V as described below, the N content is preferably set to 0.0010% or more, and more preferably 0.0020% or more, in order to promote the precipitation of carbonitrides.
O: 0.0100% or less
If O is contained in a large amount in steel, it forms coarse oxides that serve as starting points of fracture, causing brittle fracture and hydrogen induced cracking. Therefore, the O content is limited to 0.0100% or less. The O content is preferably set to 0.0080% or less and 0.0050% or less. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more or 0.0010% or more.
The remainder of the chemical composition of the hot-rolled steel sheet of the present embodiment is basically composed of Fe and impurities, and the hot-rolled steel sheet of the present embodiment may contain Nb, Ti, V, Cu, Cr, Mo, B, Ca, Mg, REM, Bi, Zr, Co, Zn, W, and Sn as optional elements in addition to the above elements. The content of the above-mentioned optional elements is 0% when not contained. The optional elements are described in detail below.
In the present embodiment, the impurities are those mixed from ores, scrap iron, manufacturing environments, and the like as raw materials, and are acceptable within a range that does not adversely affect the hot-rolled steel sheet of the present embodiment.
Nb：0～0.300％
Nb is an element that delays grain growth during hot rolling by forming carbonitride or solid-solution Nb, and contributes to improvement of low-temperature toughness through refinement of the grain size of the hot-rolled steel sheet. In the case where this effect is obtained, the Nb content is preferably set to 0.005% or more.
On the other hand, even if the Nb content exceeds 0.300%, the above effect is saturated and the economical efficiency is lowered. Therefore, even when Nb is contained as necessary, the Nb content is set to 0.300% or less.
Is selected from the group consisting of Ti: 0-0.300% and V: 0 to 0.500% of 1 or 2 of the group
Both Ti and V have the action of precipitating as carbide or nitride in steel and refining the metal structure by the pinning effect. Therefore, 1 or 2 of these elements may be contained. In order to more reliably obtain the effects of the above-described actions, it is preferable to set the Ti content to 0.005% or more or set the V content to 0.005% or more. However, even if these elements are contained in excess, the effects of the above-described actions are saturated, and this is not economically preferable. Therefore, even when it is contained, the Ti content is set to 0.300% or less, and the V content is set to 0.500% or less.
Is selected from the group consisting of Cu: 0-2.00%, Cr: 0-2.00%, Mo: 0-1.000% and B: 0 to 0.0100% of 1 or more than 2 kinds of the group
Cu, Cr, Mo and B all have the function of improving hardenability. In addition, Cr has an effect of stabilizing retained austenite, and Cu and Mo have an effect of precipitating carbide in steel to improve strength.
Cu has an effect of improving hardenability and an effect of increasing the strength of a steel sheet by precipitating as carbides in steel at low temperature. In order to more reliably obtain the effects of the above-described actions, the Cu content is preferably set to 0.01% or more, and more preferably 0.03% or more or 0.05% or more. However, if the Cu content exceeds 2.00%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less and 1.00% or less.
Cr has an effect of improving hardenability and an effect of stabilizing retained austenite. In order to more reliably obtain the effects of the above-described actions, the Cr content is preferably set to 0.01% or more or 0.05% or more. However, if the Cr content exceeds 2.00%, the chemical conversion treatability of the steel sheet is significantly reduced. Therefore, the Cr content is set to 2.00% or less.
Mo has the effect of improving hardenability and the effect of precipitating carbides in steel to improve strength. In order to more reliably obtain the effects of the above-described actions, the Mo content is preferably set to 0.010% or more or 0.020% or more. However, even if the Mo content is set to more than 1.000%, the effects of the above-described actions are saturated, and this is not economically preferable. Therefore, the Mo content is set to 1.000% or less. The Mo content is preferably 0.500% or less and 0.200% or less.
B has the function of improving hardenability. In order to more reliably obtain the effect of this action, the B content is preferably set to 0.0001% or more or 0.0002% or more. However, since the formability of the steel sheet is significantly reduced when the B content exceeds 0.0100%, the B content is set to 0.0100% or less. The content of B is preferably set to 0.0050% or less.
Is selected from the group consisting of Ca: 0-0.0200%, Mg: 0-0.0200% and REM: 0 to 0.1000% of 1 or more than 2
Ca. Both Mg and REM have the effect of improving the formability of the steel sheet by adjusting the shape of the inclusions to a preferred shape. Therefore, 1 or 2 or more of these elements may be contained. In order to more reliably obtain the effects of the above-described actions, it is preferable that the content of any one of 1 or more of Ca, Mg, and REM is set to 0.0005% or more, respectively. However, if the Ca content or the Mg content exceeds 0.0200% or the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, which may adversely decrease the formability of the steel sheet. Therefore, the Ca content and Mg content are set to 0.0200% or less, and the REM content is set to 0.1000% or less.
Here, REM means a total of 17 elements including Sc, Y, and lanthanoid, and the content of REM means a total content of these elements. In the case of lanthanides, they are added industrially in the form of mixed rare earth metals.
Bi：0～0.020％
Bi has an effect of improving formability by making the solidification structure finer, and therefore Bi may be contained in the steel. In order to more reliably obtain the effect of this action, the Bi content is preferably set to 0.0005% or more. However, even if the Bi content is set to more than 0.020%, the effects of the above-described actions are saturated, and this is not economically preferable. Therefore, the Bi content is set to 0.020% or less. The Bi content is preferably 0.010% or less.
1 or 2 or more of Zr, Co, Zn and W: 0 to 1.000% in total
Sn：0～0.050％
As to Zr, Co, Zn and W, the present inventors confirmed that: even if these elements are contained in a total amount of 1.000% or less, the effects of the hot-rolled steel sheet of the present embodiment are not impaired. Therefore, 1 or 2 or more of Zr, Co, Zn and W may be contained in a total amount of 1.000% or less.
In addition, the present inventors confirmed that: even if Sn is contained in a small amount, the effect of the hot-rolled steel sheet of the present embodiment is not impaired, but since Sn is contained, defects are likely to occur during hot rolling, the Sn content is set to 0.050% or less.
0.05％≤Si+Al≤3.00％
In the hot-rolled steel sheet according to the present embodiment, the content of each element needs to be controlled so that Si + Al satisfies the following formula (1) in addition to the above-described ranges.
Si + Al is more than or equal to 0.05% and less than or equal to 3.00% in the formula (1)
If Si + Al is less than 0.05%, scale-based defects such as scale defects and spindle scale occur.
On the other hand, if Si + Al exceeds 3.00%, the effect of improving the chemical conversion treatability and coating adhesion is not exhibited even if Ni is contained.
The contents of the respective elements in the hot rolled steel sheet described above are in accordance with JISG 1201: 2014 average content of the whole plate thickness obtained by ICP emission spectrum analysis using the cut powder.
[ metallic Structure of Steel sheet ]
Next, the metal structure (microstructure) of the hot-rolled steel sheet according to the present embodiment will be described.
In the hot-rolled steel sheet of the present embodiment, in a cross section parallel to the rolling direction of the steel sheet, the microstructure at a position 1/4 depths from the surface of the steel sheet (t/4 when the sheet thickness is t (mm)) contains, in terms of area percentage (area%), 77.0 to 97.0% in total of bainite and tempered martensite, 0 to 5.0% of ferrite, 0 to 5.0% of pearlite, 3.0% or more of retained austenite, and 0 to 10.0% of martensite, and a tensile strength of 980MPa or more and high press formability (ductility and stretch flangeability) are obtained. In the present embodiment, the reason why the metal structure at the 1/4 depth position where the distance from the steel sheet surface is the sheet thickness is defined in the cross section parallel to the rolling direction of the steel sheet is because: the metal structure at this position represents a typical metal structure of a steel sheet.
Total area fraction of bainite and tempered martensite: 77.0 to 97.0%
Bainite and tempered martensite are the most important metal structures in the present embodiment.
Bainite is a collection of lath-shaped grains. The bainite includes upper bainite, which is an aggregate of laths containing carbide between the laths, and lower bainite, which contains iron-based carbide having a major diameter of 5nm or more in the interior. The iron-based carbide precipitated in the lower bainite belongs to a single modification, i.e., an iron-based carbide group elongated in the same direction. The tempered martensite is a collection of lath-like crystal grains and contains iron-based carbides having a major axis of 5nm or more. The iron-based carbides in the tempered martensite belong to a plurality of variants, i.e., a plurality of iron-based carbide groups elongated in different directions. It is difficult to distinguish between the lower bainite and the tempered martensite by the measurement method described later, but it is not necessary to distinguish between them in the present embodiment.
As described above, bainite and tempered martensite are hard and homogeneous metal structures, and are the most suitable metal structures for making a steel sheet have both high strength and excellent stretch-flange formability. If the total area fraction of bainite and tempered martensite is less than 77.0%, the steel sheet cannot achieve both high strength and excellent stretch-flange formability. Therefore, the total area fraction of bainite and tempered martensite is set to 77.0% or more. The total area fraction of bainite and tempered martensite is preferably 85.0% or more, and more preferably 90.0% or more. Since the hot-rolled steel sheet according to the present embodiment contains 3.0% or more of retained austenite, the total area fraction of bainite and tempered martensite is 97.0% or less.
Area fraction of ferrite: 0 to 5.0 percent
Ferrite is a massive crystal grain and a metal structure including no lower structure such as laths therein. If the area fraction of the soft ferrite exceeds 5.0%, the interface between ferrite and bainite or tempered martensite, which is likely to become a starting point of generation of voids, and the interface between ferrite and retained austenite increase, and thus the stretch-flange formability of the steel sheet is particularly reduced. Therefore, the area fraction of ferrite is set to 5.0% or less. The area fraction of ferrite is preferably 4.0% or less, 3.0% or less, or 2.0% or less. In order to improve stretch flangeability of the steel sheet, the area fraction of ferrite is preferably reduced as much as possible, and the lower limit thereof is set to 0%.
Area fraction of pearlite: 0 to 5.0 percent
Pearlite is a lamellar metal structure in which cementite is precipitated in layers between ferrite and bainite, and is a softer metal structure than bainite. When the area fraction of pearlite exceeds 5.0%, the interfaces between pearlite and bainite or tempered martensite, which are likely to become starting points for the generation of voids, and the interfaces between pearlite and retained austenite increase, and therefore the stretch-flangeability of the steel sheet in particular decreases. Therefore, the area fraction of pearlite is set to 5.0% or less. The area fraction of pearlite is preferably 4.0% or less, 3.0% or less, or 2.0% or less. In order to improve the stretch flangeability of the steel sheet, the area fraction of pearlite is preferably reduced as much as possible, and the lower limit thereof is set to 0%.
Area fraction of martensite: 0 to 10.0%
In the present embodiment, martensite is defined as a metal structure in which carbides having a diameter of 5nm or more are not precipitated between laths and in laths. Martensite (so-called fresh martensite) is a very hard structure and contributes greatly to the increase in strength of the steel sheet. On the other hand, if martensite is included, the interface between martensite and bainite and tempered martensite, which are the parent phase, becomes a starting point for generation of voids, and particularly the stretch-flange formability of the steel sheet is reduced. Further, since martensite is a hard structure, the low-temperature toughness of the steel sheet deteriorates. Therefore, the area fraction of martensite is set to 10.0% or less. Since the hot-rolled steel sheet according to the present embodiment contains predetermined amounts of bainite and tempered martensite, a desired strength can be ensured even when martensite is not contained. In order to obtain the desired stretch-flange formability of the steel sheet, the area fraction of martensite is preferably reduced as much as possible, and the lower limit thereof is set to 0%.
The bainite, tempered martensite, ferrite, pearlite, and martensite constituting the metal structure of the hot-rolled steel sheet according to the present embodiment as described above are identified, the existence position is confirmed, and the area fraction is measured by the following method.
First, a cross section parallel to the rolling direction of a steel sheet is etched using a nital reagent and a reagent disclosed in jp 59-219473 a. Specifically, regarding the corrosion of the cross section, a solution obtained by dissolving 1 to 5g of picric acid in 100ml of ethanol is referred to as a solution A, a solution obtained by dissolving 1 to 25g of sodium thiosulfate and 1 to 5g of citric acid in 100ml of water is referred to as a solution B, and the ratio of the solution A to the solution B is set to 1: 1 to obtain a mixed solution, and adding nitric acid in a ratio of 1.5 to 4% relative to the total amount of the mixed solution and mixing to obtain a solution as a pretreatment solution. The post-treatment solution was prepared by adding the pre-treatment solution to a 2% nital solution in an amount of 10% based on the total amount of the 2% nital solution and mixing the resulting mixture. The cross section parallel to the rolling direction of the steel sheet is immersed in the pretreatment liquid for 3 to 15 seconds, washed with alcohol and dried, and then immersed in the post-treatment liquid for 3 to 20 seconds, washed with water and dried, thereby corroding the cross section.
Then, at least 3 regions of 40 μm × 30 μm were observed at a depth of 1/4 mm from the surface of the steel sheet at a magnification of 1000 to 100000 times using a scanning electron microscope, and each phase in the metal structure was identified based on whether or not the above-described characteristics were included, and the existence position of each phase and the area fraction were confirmed.
Area fraction of retained austenite: 3.0% or more
The retained austenite is a metal structure that exists as a face-centered cubic lattice even at room temperature. The retained austenite has an effect of improving ductility of the steel sheet by transformation induced plasticity (TRIP). If the area fraction of the retained austenite is less than 3.0%, the effects of the above-described actions are not obtained, and the ductility of the steel sheet deteriorates. Therefore, the area fraction of retained austenite is set to 3.0% or more. The area fraction of retained austenite is preferably 5.0% or more, more preferably 7.0% or more, and still more preferably 8.0% or more. The upper limit of the area fraction of retained austenite is not particularly limited, but the area fraction of retained austenite that can be ensured in the chemical composition of the hot-rolled steel sheet according to the present embodiment is approximately 20.0% or less, and therefore the upper limit of the area fraction of retained austenite may be set to 20.0%.
The method of measuring the surface integral ratio of retained austenite includes methods using X-ray Diffraction, EBSP (Electron Back Scattering Diffraction Pattern) analysis, magnetic measurement, and the like, and the measured value may vary depending on the measurement method. In the present embodiment, the area fraction of retained austenite is measured by X-ray diffraction.
In the measurement of the retained austenite surface integral fraction by X-ray diffraction in the present embodiment, first, the integrated intensities of 6 peaks in total of α (110), α (200), α (211), γ (111), γ (200), and γ (220) are obtained using Co — K α rays in a cross section parallel to the rolling direction of the steel sheet at the 1/4 depth position of the steel sheet thickness, and are calculated by an intensity averaging method, thereby obtaining the volume fraction of the retained austenite. The volume fraction and the area fraction were set to be equal to each other, and the area fraction was set to be the area fraction of retained austenite.
In the present embodiment, since the area fractions of bainite, tempered martensite, ferrite, pearlite, and martensite (area fractions other than retained austenite) and the area fraction of retained austenite are measured by different measurement methods, the total of the area fractions of the two may not be 100.0%. When the total of the area fractions of the retained austenite and the other austenite is not 100.0%, the area fractions of the retained austenite and the other austenite are adjusted so that the total becomes 100.0%. For example, when the total of the area fraction of the retained austenite and the area fraction of the retained austenite is 101.0%, a value obtained by multiplying the area fraction of the retained austenite other than the retained austenite by 100.0/101.0 obtained by the measurement is defined as the area fraction of the retained austenite other than the retained austenite, and a value obtained by multiplying the area fraction of the retained austenite by 100.0/101.0 obtained by the measurement is defined as the area fraction of the retained austenite, so that the total of the two is 100.0%.
When the total of the area fraction other than the retained austenite and the area fraction of the retained austenite is less than 95.0% or exceeds 105.0%, the area fraction is measured again.
Average crystal grain size of the metal structure other than the retained austenite: 7.0 μm or less
The low-temperature toughness is improved by making the average grain size (hereinafter, simply referred to as the average grain size) of the microstructure other than the retained austenite (bainite, tempered martensite, ferrite, pearlite, and martensite as main phases) fine. When the average crystal grain size exceeds 7.0. mu.m, vTrs is not more than-50 ℃ which is an index of low-temperature toughness required for steel sheets for automobile chassis parts. Therefore, the average crystal grain size is set to 7.0 μm or less. The lower limit of the average crystal grain size is not particularly limited, but the smaller the average crystal grain size, the more preferable the average crystal grain size is, the larger the average crystal grain size may be, the larger the average crystal grain size is. However, it may be practically difficult from the viewpoint of manufacturing facilities to set the average crystal grain size to less than 1.0 μm, and therefore the average crystal grain size may be set to 1.0 μm or more.
In the present embodiment, EBSP-OIM is used as the crystal grainsTM(Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method. In the EBSP-OIM method, a high-gradient sample is irradiated with an electron beam in a Scanning Electron Microscope (SEM), a high-sensitivity camera is used to photograph a Kikuchi pattern formed by back scattering, and the photographed photograph is subjected to image processing by a computer, whereby the crystal orientation of the irradiated point can be measured in a short time. The EBSP-OIM method is performed using an apparatus in which a scanning electron microscope and an EBSP analyzer are combined, and OIM Analysis (registered trademark) manufactured by AMETEK corporation. In the EBSP-OIM method, the microstructure and crystal orientation of the sample surface can be quantitatively analyzed. In addition, the analyzable region in the EBSP-OIM method is a region that can be observed by SEM. Although the resolution also varies depending on SEM, according to the EBSP-OIM method, analysis can be performed at a resolution of 20nm as a minimum. Since it is generally recognized that the threshold value of the high angle grain boundary of the grain boundary is 15 °, in the present embodiment, the grain is visualized by an image mapped by defining the adjacent grain having the orientation difference of 15 ° or more as one grain, and the average grain size of the area average calculated by OIM Analysis is obtained.
When measuring the average crystal grain size of the microstructure at a position 1/4 depth from the surface of the steel sheet in a cross section parallel to the rolling direction of the steel sheet, the average crystal grain size is determined by measuring at least 10 visual fields in a region of 40 μm × 30 μm at a magnification of 1200 times, and the average of the grain sizes (effective crystal grain sizes) of crystals having a misorientation of adjacent crystal grains of 15 ° or more is set as the average crystal grain size. In the present measurement method, the microstructure other than the main phase is judged to have little influence because of a small area fraction, and the average grain size of bainite and tempered martensite as the main phase is not distinguished from the average grain size of ferrite, pearlite, and martensite. That is, the average grain size measured by the above-described measurement method is the average grain size of bainite, tempered martensite, ferrite, pearlite, and martensite. In the measurement of the effective crystal grain size of pearlite, the effective crystal grain size of ferrite in pearlite is measured, not the effective crystal grain size of pearlite block.
Since the crystal structure of the retained austenite is FCC and the other microstructures are BCC, which are different from each other, the average grain size of the metal structure other than the retained austenite can be easily measured by EBSP.
Average number density of iron-based carbides having a diameter of 20nm or more: 1.0X 106Per mm2The above
The steel content is 1.0X 106Per mm2The reason why the iron-based carbide having a diameter of 20nm or more is described above is that: the low-temperature toughness of the parent phase is improved, and the balance between the excellent strength and the low-temperature toughness is obtained. The iron-based carbide in the present embodiment is an iron-based carbide containing Fe and C and having a major axis length of less than 1 μm. That is, cementite in pearlite having a major axis length of 1 μm or more or coarse carbide precipitated between bainite laths is not targeted in the present embodiment. In the case where the matrix phase is martensite in a quenched state, the strength is excellent, but the low-temperature toughness is insufficient, and therefore, it is necessary to improve the low-temperature toughness. Then, by precipitating iron-based carbides of a predetermined number or more in the steel by tempering or the like, the low-temperature toughness of the main phase is improved, and the low-temperature toughness (vTrs. ltoreq. -50 ℃) required for steel sheets for automobile chassis parts is achieved.
The present inventors investigated the relationship between the low-temperature toughness of steel sheets and the number density of iron-based carbides, and found that: by setting the number density of iron-based carbides in the metal structure to 1.0X 106Per mm2In particular, the number density of iron-based carbides in tempered martensite and lower bainite is set to 1.0X 106Per mm2As described above, excellent low-temperature toughness can be obtained. Therefore, in the present embodiment, in the metal structure at the 1/4-depth position from the steel sheet surface to the sheet thickness in the cross section parallel to the rolling direction of the steel sheet, the number density of the iron-based carbides is set to 1.0 × 106Per mm2The above. The number density of the iron-based carbide is preferably 5.0X 106Per mm2Above, more preferably 1.0 × 107Per mm2The above.
It is also presumed that the size of the iron-based carbide precipitated in the hot-rolled steel sheet according to the present embodiment is as small as 300nm or less, and most of the iron-based carbide precipitates in the laths of martensite and bainite, so that the low-temperature toughness is not deteriorated.
The number density of iron-based carbides is measured by collecting a sample with a cross section parallel to the rolling direction of the steel sheet as an observation plane, polishing the observation plane, etching the steel sheet with nital, and observing the steel sheet in a range of 1/8 to 3/8 thickness centered at 1/4 depth from the surface of the steel sheet with a Field Emission Scanning Electron Microscope (FE-SEM). The number density of iron-based carbides having a diameter of 20nm or more was measured by observing the iron-based carbides with a magnification of 200000 times in 10 visual fields or more.
Average Ni concentration in the surface: 7.0% or more
In order to obtain excellent chemical conversion treatability and coating adhesion of the zirconium-based chemical conversion treatment coating film also on the surface of the ultrahigh-strength steel sheet after pickling (before chemical conversion treatment), it is preferable to reduce oxides of Si, Al, and the like on the pickled sheet surface to harmless levels. In order to obtain the above-described effects by controlling only oxides of Si, Al, and the like, it is necessary to set the preheating zone of the heating furnace to use Ar, He, and N in order to suppress oxidation of the slab surface as much as possible in the heating step of hot rolling2And the like, substantially non-oxidizing atmosphere of inert gas, or incomplete combustion in which the air ratio is set to less than 0.9. However, when it is assumed that the production of a hot-rolled steel sheet is carried out at low cost in a general process for producing a hot-rolled steel sheet and in mass production, it is impossible to set a substantially non-oxidizing atmosphere using an inert gas in a heating process for hot rolling. In addition, if the air ratio is set to less than 0.9 in order to control oxides of Si, Al, and the like, there are problems that heat loss due to incomplete combustion is significantly increased, thermal efficiency of the heating furnace itself is lowered, and production cost is increased.
The present inventors have studied the adhesion of a coating film after chemical conversion treatment using a zirconium-based chemical conversion treatment liquid on an ultrahigh-strength steel sheet having the above chemical components and structure, tensile strength of 980MPa or more, and excellent ductility and stretch-flange formability, on the premise that a production process that is inexpensive and mass-producible is applied. In general, since the hot-rolled steel sheet is subjected to chemical conversion treatment after pickling, the steel sheet after pickling is also evaluated in the present embodiment. In the present embodiment, the pickling is performed using a 1-10 mass% hydrochloric acid solution at a temperature of 20-95 ℃ for a pickling time of 30 seconds or more and less than 60 seconds. When no scale was formed on the surface, evaluation was performed without pickling.
The results of the study know: in the measurement using FE-EPMA, if the average Ni concentration in the surface is 7.0% by mass or more, even if oxides of Si, Al, and the like remain on the surface of the pickled plate, the coating peel width evaluated by the method described later is within 4.0mm as a standard in all samples, and the coating adhesion is excellent. In this case, an uncovered portion was not observed in the chemical conversion coating. On the other hand, the coating peel width exceeded 4.0mm in all samples in which the average Ni concentration in the surface was less than 7.0%.
This is believed to be due to: as shown in fig. 2, by forming the Ni concentrated portion 3 on the surface of the steel sheet, a potential difference is generated between the Ni locally concentrated on the surface and the base metal 1, and the Ni forms a precipitation nucleus of the zirconium-based chemical conversion crystal, thereby promoting the formation of the zirconium-based chemical conversion crystal 4. The base metal 1 refers to a portion of the steel sheet other than the scale 2.
Therefore, in the hot-rolled steel sheet according to the present embodiment, the average Ni concentration in the surface (surface after pickling and before chemical conversion treatment) is 7.0% or more. If the average Ni concentration on the surface is 7.0% or more, even if oxides such as Si and Al remain on the surface, they are sufficient for forming precipitation nuclei of the zirconium-based chemical conversion crystal. In order to set the average Ni concentration in the surface to 7.0% or more, it is necessary to selectively oxidize Fe to some extent in the surface of the steel sheet in the heating step of hot rolling, thereby making Ni, which is less likely to be oxidized than Fe, concentrated on the base metal side of the interface between the scale and the base metal.
The average Ni concentration of the steel sheet surface was measured using a JXA-8530F field emission electron probe microanalyzer (FE-EPMA). The measurement conditions were: acceleration voltage: 15kV, irradiation current: 6X 10-8A. Irradiation time: 30ms, beam diameter: 1 μm. The measurement was carried out with respect to a measurement area of 900 μm from a direction perpendicular to the surface of the steel sheet2In the above, the Ni concentration in the measurement range is averaged (Ni concentration at all measurement points is averaged).
Fig. 1 shows an example of the EPMA measurement result of the surface.
Ni is mainly concentrated on the base metal side at the interface between the scale and the base metal. In addition, before the chemical conversion treatment, an acid washing is generally performed. Therefore, when the target steel sheet has a scale formed on the surface thereof, the measurement is performed after pickling as in the case of the steel sheet subjected to the chemical conversion treatment.
The coating adhesion of the above-mentioned pickled plate was evaluated by the following procedure. First, the produced steel sheet is subjected to acid treatment, and then to chemical conversion treatment for adhering a zirconium-based chemical conversion treatment film. Further, after the upper surface was subjected to electrodeposition coating with a thickness of 25 μm and subjected to coating sintering treatment at 170 ℃ for 20 minutes, the electrodeposition coating film was scribed with a knife with a pointed tip into a slit having a length of 130mm until reaching the base metal. Then, the measured value was measured in accordance with JIS Z2371: 2015, spraying 5% saline at 35 ℃ for 700 hours, sticking a 24 mm-wide tape (NICIBAN 405A-24JIS Z1522: 2009) to the cut part in parallel with the cut part at a length of 130mm, and measuring the maximum peeling width of the coating film when peeled.
An internal oxide layer (a region in which oxide is generated inside a base metal) is present in the hot-rolled steel sheet, and the average depth of the internal oxide layer from the surface of the hot-rolled steel sheet is 5.0 μm or more and 20.0 μm or less
Even if the Ni-concentrated portion exists in the surface layer, if the coating ratio of oxides of Si, Al, and the like is excessively large in the surface of the hot-rolled steel sheet, an "uncoated portion" to which the zirconium-based chemical conversion coating film is not attached is likely to be generated. In order to suppress this, it is preferable to set the oxidation of Si, Al, or the like to: instead of external oxidation, which forms an oxide on the outside as compared with the base metal, internal oxidation, which forms an oxide on the inside, is performed.
The inventors of the present invention examined the relationship between the width of the coating peeling and the average depth of the internal oxide layer from the surface of the steel sheet (average of the positions of the lower ends of the internal oxide layers) only by observing the cross section of a sample having an average Ni concentration of 7.0% or more in the surface with an optical microscope. As a result, the coating peel width of all the samples having an average depth of the internal oxidation layer of 5.0 μm or more was within 3.5mm, whereas the coating peel width of all the samples having an average depth of the internal oxidation layer of less than 5.0 μm was more than 3.5mm and 4.0mm or less.
Therefore, in order to obtain more excellent coating adhesion, it is preferable to set the average depth of the internal oxide layer from the surface of the hot-rolled steel sheet to 5.0 μm or more and 20.0 μm or less.
When the average depth of the internal oxide layer of Si, Al, or the like is less than 5.0 μm, the effect of suppressing the "uncoated portion" to which the zirconium-based chemical conversion coating film is not attached is small. On the other hand, if the average depth exceeds 20.0 μm, not only the effect of inhibiting the "uncoated portion" to which the zirconium-based chemical conversion coating film is not attached is saturated, but also the hardness of the surface layer may be lowered by the formation of a decarburized layer due to the internal oxidation, and the fatigue durability may be deteriorated.
The average depth of the internal oxide layer was measured by cutting a surface parallel to the rolling direction and the plate thickness direction at a position 1/4 or 3/4 in the plate width direction of the pickled plate as an embedding sample, embedding the sample in a resin sample, then performing mirror polishing, and observing the sample with an optical microscope in a field of view of 195 μm × 240 μm (corresponding to 400 times magnification) without etching for 12 or more fields of view. When a straight line is drawn in the thickness direction, the position intersecting the surface of the steel sheet is set as the surface, the depth (position of the lower end) of the internal oxide layer in each visual field with respect to the surface is measured at 5 points for each 1 visual field, and the average value is calculated by removing the maximum value and the minimum value from the average values in each visual field, and the average value is set as the average depth of the internal oxide layer.
Standard deviation of arithmetic average roughness Ra of the surface of hot-rolled steel sheet after pickling under prescribed conditions: 10.0 to 50.0 μm in thickness
The zirconium-based chemical conversion coating is very thin, on the order of several tens of nm, compared to a conventional zinc phosphate coating having a thickness of several μm. The difference in film thickness is caused by the fact that the zirconium-based chemical conversion treatment crystals are very fine. If the chemical conversion treatment crystals are fine, the chemical conversion treatment surface is very smooth, and therefore it is difficult to obtain strong adhesion to the coating film due to the anchor effect seen in the zinc phosphate treatment coating.
However, the results of the studies carried out by the present inventors have revealed that: if the steel sheet surface is formed with irregularities, the adhesion between the chemical conversion coating and the coating film can be improved.
Based on the findings, the inventors investigated the relationship between the standard deviation of the arithmetic mean roughness Ra of the surface of the pickled plate before the zirconium-based chemical conversion treatment and the coating adhesion, for samples having an average Ni concentration of 7.0% or more and an average depth of the internal oxide layer of 5.0 μm or more. As a result, the coating peel width of all samples having an arithmetic mean roughness Ra of 10.0 μm or more and 50.0 μm or less on the surface of the pickled plate was within 3.0mm, whereas the coating peel width of all samples having an arithmetic mean roughness Ra of less than 10.0 μm or more and more than 50.0 μm on the surface of the pickled plate was within 3.0mm and 3.5 mm.
Therefore, the standard deviation of the arithmetic average roughness Ra of the surface of the steel sheet after pickling is preferably 10.0 μm or more and 50.0 μm or less.
When the standard deviation of the arithmetic average roughness Ra of the steel sheet surface is less than 10.0 μm, a sufficient anchoring effect cannot be obtained. On the other hand, when the standard deviation of the arithmetic mean roughness Ra of the steel sheet surface after pickling exceeds 50.0 μm, not only the anchor effect is saturated, but also zirconium-based chemical conversion treatment crystals are less likely to adhere to the side surfaces of the uneven valleys or ridges of the steel sheet surface after pickling, and "uncoated portions" are likely to occur.
The roughness of the surface of the steel sheet varies greatly depending on the pickling conditions, but in the hot-rolled steel sheet of the present embodiment, the standard deviation of the arithmetic average roughness Ra of the surface of the hot-rolled steel sheet after pickling is performed using a 1-10 mass% hydrochloric acid solution at a temperature of 20-95 ℃ for a pickling time of 30 seconds or more and less than 60 seconds is preferably 10.0 μm or more and 50.0 μm or less.
The standard deviation of the arithmetic average roughness Ra was determined by JIS B0601: 2013, and the surface roughness of the pickled plate is measured. After the arithmetic average roughness Ra of the front and back surfaces of 12 samples or more were measured, the standard deviation of the arithmetic average roughness Ra of each sample was calculated, and the average value was calculated by excluding the maximum value and the minimum value among the standard deviations.
The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, and may be set to 0.8 to 8.0 mm. When the thickness of the steel sheet is less than 0.8mm, it becomes difficult to secure the rolling completion temperature, and the rolling load becomes too large, so that hot rolling may become difficult. Therefore, the thickness of the steel sheet of the present invention may be set to 0.8mm or more. More preferably 1.2mm or more, and still more preferably 1.4mm or more. On the other hand, if the thickness exceeds 8.0mm, it may be difficult to refine the metal structure and to ensure the steel structure. Therefore, the plate thickness may be set to 8.0mm or less. More preferably 6.0mm or less.
The hot-rolled steel sheet of the present embodiment having the above-described chemical composition and metal structure may be surface-treated to have a plated layer on the surface thereof for the purpose of improving corrosion resistance. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of the plating layer include a zinc plating layer and a Zn — Ni alloy plating layer. Examples of the hot-dip coating layer include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum layer, a hot-dip Zn — Al alloy layer, a hot-dip Zn — Al — Mg alloy layer, and a hot-dip Zn — Al — Mg — Si alloy layer. The plating deposition amount is not particularly limited, and may be set in the same manner as in the conventional art. After plating, appropriate chemical conversion treatment (for example, coating and drying of a silicate-based chromium-free chemical conversion treatment liquid) may be performed to further improve the corrosion resistance.
[ production method ]
The hot-rolled steel sheet of the present embodiment having the above-described chemical composition and metal structure can be produced by the following production method.
In order to obtain the hot-rolled steel sheet according to the present embodiment, it is important to perform heating under a predetermined condition, to perform hot rolling, and to perform accelerated cooling to a predetermined temperature range, and to control the cooling history of the outermost periphery of the coil and the inside of the coil after winding. In addition, when heating a slab before hot rolling, it is important to control the air ratio in the heating furnace.
In the method for producing a hot-rolled steel sheet according to the present embodiment, the following steps (I) to (VI) are sequentially performed. The slab temperature and the steel sheet temperature in the present embodiment refer to the slab surface temperature and the steel sheet surface temperature.
(I) The slab is heated to a temperature above 1150 ℃.
(II) hot rolling is performed so that the cumulative reduction ratio is 90% or more in total in a temperature range of 850 to 1100 ℃ and so that the finish rolling temperature is equal to or higher than a temperature T2 (DEG C) represented by the following formula (2).
(III) starting cooling within 1.5 seconds after completion of hot rolling, and accelerating cooling at an average cooling rate of 50 ℃/second or more to a temperature T3 (. degree. C.) or less represented by the following formula (3).
(IV) cooling from the cooling stop temperature of the accelerated cooling to the coiling temperature at an average cooling rate of 10 ℃/sec or more.
(V) is taken up at (T4-100) DEG C to (T4+50) DEG C with respect to a temperature T4 (DEG C) represented by the following formula (4).
T2(℃)＝868-396×[C]-68.1×[Mn]+24.6×[Si]-36.1×[Ni]-24.8×[Cr]-20.7×[Cu]+250×[Al] (2)
T3(℃)＝770-270×[C]-90×[Mn]-37×[Ni]-70×[Cr]-83×[Mo] (3)
T4(℃)＝591-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo] (4)
Wherein [ element symbol ] in each formula represents the content (mass%) of each element in the steel slab.
The contents of each element of the steel slab were obtained by using spark discharge emission spectroscopy (Quantvac, QV) on a sample collected from the molten steel.
[ heating Process ]
The slab (billet) to be subjected to hot rolling may be a slab obtained by continuous casting, a slab obtained by casting or cogging, or the like, and a slab obtained by hot working or cold working thereof as necessary may be used.
From the viewpoint of Ni thickening in the slab surface, an increase in rolling load during hot rolling, and a concomitant deterioration in material quality due to insufficient cumulative reduction in the slab interior, the temperature of the slab subjected to hot rolling (slab heating temperature) is set to 1150 ℃ or higher. From the viewpoint of suppressing the scale loss, the slab heating temperature is preferably set to 1350 ℃. When the slab to be subjected to hot rolling is a slab obtained by continuous casting or a slab obtained by cogging rolling and is in a high temperature state (1150 ℃ or higher), it may be subjected to hot rolling without heating.
However, in order to obtain excellent coating film adhesion, it is important to control the air ratio of each zone of the heating furnace in slab heating as follows. In order to control the air ratio of each zone, the burner device of the heating furnace is preferably set as a regenerative burner. This is due to: the regenerative burner has higher heat uniformity of the temperature in the furnace and higher controllability of each zone than the conventional burner, and particularly, the air ratio in each zone can be strictly controlled, so that the control of the heating furnace described later can be realized.
Preferred air ratios of the respective zones will be described.
< air ratio in preheating zone: 1.1 to 1.9>
By setting the air ratio in the preheating zone to 1.1 or more, the surface of the hot-rolled steel sheet after pickling can be enriched with Ni and the average Ni concentration can be set to 7.0% or more.
When the behavior of scale growth on the surface of a slab in a heating furnace is evaluated as the thickness of the scale to be formed, it is classified into a linear rule, which is the control of the oxygen supply rate from the atmosphere in the surface of the slab, and a parabolic rule, which is the control of the diffusion rate of iron ions in the scale, according to the air ratio (oxygen partial pressure) thereof. In order to promote the growth of the scale of the slab to some extent within a limited in-furnace time in the heating furnace and form a sufficiently dense Ni layer on the surface layer, the growth of the scale thickness needs to follow the parabolic law.
If the air ratio in the preheating zone is less than 1.1, the growth of the scale does not become the parabolic rule, and a sufficient Ni-concentrated layer cannot be formed on the surface layer of the slab within a limited in-furnace time in the heating furnace. In this case, the average Ni concentration in the surface of the hot-rolled steel sheet after pickling is not 7.0% or more, and good coating adhesion cannot be obtained.
On the other hand, if the air ratio in the preheating zone exceeds 1.9, not only will the amount of scale falling increase and the yield rate deteriorate, but also the heat loss due to the increase in exhaust gas will increase and the thermal efficiency deteriorates and the production cost increases.
The amount of scale formed in the heating furnace is controlled by the atmosphere in the preheating zone immediately after the insertion of the heating furnace, and the thickness of the scale is not substantially affected even if the atmosphere in the subsequent zone changes thereafter. Therefore, control of the scale growth behavior in the preheating zone is very important.
< air ratio in heating zone: 0.9 to 1.3>
For the formation of the internal oxide layer, it is necessary to control the air ratio in the heating zone in the heating step, and the average depth of the internal oxide layer can be set to 5.0 to 20.0 μm by setting the air ratio in the heating zone to 0.9 or more and 1.3 or less.
If the air ratio in the heating zone is less than 0.9, the average depth of the internal oxide layer is not less than 5.0. mu.m. On the other hand, if the air ratio in the heating zone exceeds 1.3, not only the average depth of the internal oxide layer becomes more than 20.0 μm, but also the hardness of the surface layer may be lowered by the formation of a decarburized layer, and fatigue durability may be deteriorated.
< air ratio in soaking zone: 0.9 to 1.9>
In order to control the unevenness of the surface of the steel sheet after pickling, it is effective to control the air ratio in the soaking zone, which is the zone immediately before the drawing-out in the heating step. Ni, which is harder to oxidize than Fe in the preheating zone, is concentrated on the base metal side of the interface between the scale and the base metal. By the Ni-concentrated layer having this Ni-concentrated portion, oxidation is suppressed in the surface layer, but external oxidation is suppressed in the next heating zone, promoting internal oxidation. Thereafter, by controlling the air ratio in the soaking zone, for example, as shown in fig. 3, the degree of oxidation of the interface between the scale 2 and the base metal 1 becomes uneven due to the scale 2 eroding to the crystal grain boundary 5 or the like, which is easy to diffuse, or due to the difference in Ni concentration on the surface of the base metal 1 caused by the difference in the concentration of Ni or the like, so that the unevenness of the interface between the scale 2 and the base metal 1 becomes large. Although not shown in fig. 3, the Ni-enriched portions 3 around the internal oxide 6 suppress erosion of the grain boundaries due to the scale 2, and also cause unevenness. When this steel sheet is pickled, the scale 2 is removed, and the surface of the hot-rolled steel sheet has a predetermined roughness.
By setting the air ratio in the soaking zone to 0.9 or more and 1.9 or less, the standard deviation of the arithmetic mean roughness Ra of the surface of the hot-rolled steel sheet after hot rolling, which is pickled with a 1-10 mass% hydrochloric acid solution at a temperature of 20-95 ℃ for a pickling time of 30 seconds or more and less than 60 seconds, can be set to 10.0 μm or more and 50.0 μm or less, for example.
If the air ratio in the soaking zone is less than 0.9, the oxygen potential of only the nuclei of the oxide selectively formed in the crystal grain boundary where diffusion is easy cannot be achieved. Therefore, the standard deviation of the arithmetic average roughness Ra of the surface of the steel sheet after pickling is less than 10.0 μm or more. On the other hand, when the air ratio in the soaking zone exceeds 1.9, the depth of the selectively oxidized crystal grain boundary in the thickness direction becomes too deep, and the standard deviation of the arithmetic mean roughness Ra of the steel sheet surface after pickling becomes more than 50.0 μm.
Air ratio of preheating zone > air ratio of heating zone
Control of the air ratio in the preheating zone is important in order to control the Ni concentration on the surface of the hot-rolled steel sheet after pickling. On the other hand, control of the air ratio in the heating zone is important in order to control the degree of formation of the internal oxide layer. Therefore, it is necessary to promote the growth of the scale of the slab to some extent in the limited in-furnace time in the preheating zone to form a sufficiently Ni-concentrated layer on the surface layer. Therefore, the growth of the scale thickness requires a relatively high air ratio according to the parabolic rule. On the other hand, in order to control the average depth of the internal oxide layer to a preferable range, it is necessary to suppress the air ratio in the heating zone to be relatively low and suppress the rapid growth of the internal oxide layer. In addition, if the air ratio in the heating zone is high, a decarburized layer may be formed and grown to lower the hardness of the surface layer, thereby deteriorating the fatigue durability. Therefore, the air ratio of the preheating zone is preferably set to be higher than that of the heating zone.
[ Hot Rolling Process ]
The hot rolling is preferably performed by a reversing mill or a tandem mill as the multi-pass rolling. In particular, from the viewpoint of industrial productivity, at least the final stages are more preferably hot rolling using a tandem mill.
Reduction of hot rolling: a cumulative reduction (reduction in sheet thickness) of 90% or more in total in a temperature range of 850 to 1100 DEG C
By hot rolling at a temperature of 850 to 1100 ℃ so that the cumulative reduction ratio becomes 90% or more in total, it is possible to refine the average crystal grain size of bainite and tempered martensite which are main phases while mainly refining recrystallized austenite grains and promoting accumulation of strain energy into unrecrystallized austenite grains. Therefore, hot rolling is performed at a temperature of 850 to 1100 ℃ so that the cumulative reduction (the reduction in sheet thickness by rolling is 90% or more) becomes 90% or more in total. The cumulative reduction in the temperature range of 850 to 1100 ℃ is a percentage of the difference between the inlet plate thickness before the first pass in the rolling in the temperature range and the outlet plate thickness after the final pass in the rolling in the temperature range.
Hot rolling completion temperature (finish rolling temperature): t2 (DEG C) or more
The finishing temperature of hot rolling is set to T2 (DEG C) or higher. By setting the finishing temperature of hot rolling to T2 (c) or higher, an excessive increase in the ferrite nucleus generation sites in austenite can be suppressed, and the area fraction of ferrite in the final structure (the microstructure of the hot-rolled steel sheet after production) can be suppressed to less than 5.0%.
[ Primary Cooling Process ]
Accelerated cooling after completion of hot rolling: cooling was started within 1.5 seconds, and the mixture was cooled to T3 (. degree.C.) at an average cooling rate of 50 ℃ per second or more
In order to suppress the growth of austenite grains that are made fine by hot rolling, accelerated cooling is started within 1.5 seconds after completion of hot rolling.
Accelerated cooling (primary cooling) is started within 1.5 seconds after completion of hot rolling, and cooling is performed at an average cooling rate of 50 ℃/sec or more to T3 (c) or less, whereby generation of ferrite and pearlite is suppressed, and the area fraction of bainite and tempered martensite is increased. This improves the uniformity of the metal structure, and improves the strength and stretch flangeability of the steel sheet. The average cooling rate here is a value obtained by dividing the temperature decrease range of the steel sheet from the start of accelerated cooling (when the steel sheet is introduced into the cooling equipment) to the completion of accelerated cooling (when the steel sheet is discharged from the cooling equipment) by the time required from the start of accelerated cooling to the end of accelerated cooling. In the accelerated cooling after completion of hot rolling, if the time until the start of cooling exceeds 1.5 seconds, the average cooling rate is less than 50 ℃/sec, or the cooling stop temperature exceeds T3(° c), ferrite transformation and/or pearlite transformation in the steel sheet becomes remarkable, and it becomes difficult to obtain a microstructure mainly composed of bainite and tempered martensite. Therefore, accelerated cooling after completion of hot rolling starts cooling within 1.5 seconds after completion of hot rolling, and is cooled to T3 (DEG C) or less at an average cooling rate of 50 ℃/sec or more. The upper limit of the cooling rate is not particularly limited, but if the cooling rate is increased, the cooling facility becomes large in size, and the facility cost becomes high. Therefore, considering the facility cost, the average cooling rate is preferably 300 ℃/sec or less. The cooling stop temperature of the accelerated cooling is preferably set to (T4-100) ℃ or higher.
[ Secondary Cooling Process ]
Average cooling rate from cooling stop temperature of primary cooling to winding temperature: 10 ℃/second or more
In order to suppress the area fraction of pearlite to less than 5.0%, the average cooling rate from the cooling stop temperature of the accelerated cooling to the coiling temperature is set to 10 ℃/sec or more (secondary cooling). This increases the area fraction of bainite and tempered martensite, and improves the balance between the strength and stretch-flange formability of the steel sheet. The average cooling rate here is a value obtained by dividing the temperature decrease range of the steel sheet from the cooling stop temperature of the accelerated cooling to the coiling temperature by the time required from the stop of the accelerated cooling to the coiling. When the average cooling rate is less than 10 ℃/sec, the area fraction of pearlite increases, the strength decreases, and the ductility decreases. Therefore, the average cooling rate from the cooling stop temperature of the accelerated cooling to the winding temperature is set to 10 ℃/sec or more. The upper limit is not particularly limited, but considering the warpage of the sheet due to thermal strain, the average cooling rate is preferably 300 ℃/sec or less.
[ coiling Process ]
Coiling temperature: (T4-100) deg.C to (T4+50) deg.C
The coiling temperature is set to (T4-100) DEG C to (T4+50) DEG C. When the coiling temperature is set to be lower than (T4-100) ° c, the discharge of carbon from bainite and tempered martensite into austenite does not proceed, and the austenite is not stabilized, so that it becomes difficult to obtain 3.0% or more of retained austenite in terms of area fraction, and the ductility of the steel sheet decreases. In addition, the number density of the iron-based carbides is also reduced, and the low-temperature toughness of the steel sheet is also deteriorated. Further, when the coiling temperature exceeds (T4+50) ° C, carbon discharged from bainite and tempered martensite precipitates excessively in the steel as iron-based carbides, so that the carbon is not sufficiently concentrated in austenite, which is also disadvantageous in setting the C concentration in the retained austenite to 0.50 mass% or more. Therefore, the winding temperature is set to (T4-100) DEG C to (T4+50) DEG C.
After coiling, the steel sheet may be cooled to room temperature by a usual method.
[ Pickling step ]
[ skin pass rolling Process ]
For the purpose of improving ductility by straightening the shape of the steel sheet or introducing mobile dislocations, skin pass rolling may be performed at a reduction ratio of 0.1% to 2.0%. The hot-rolled steel sheet thus obtained may be pickled as necessary for the purpose of removing scale adhering to the surface of the hot-rolled steel sheet. When the acid washing is performed, the acid washing is preferably performed using a 1 to 10 wt% hydrochloric acid solution at a temperature of 20 to 95 ℃ for a washing time of 30 seconds or more and less than 60 seconds.
Further, after pickling, skin pass rolling or cold rolling may be performed on the obtained hot-rolled steel sheet at a reduction ratio of 10% or less on-line or off-line.
According to the above manufacturing method, the hot-rolled steel sheet according to the present embodiment can be manufactured.
Examples
The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples.
Steels having the composition shown in steel nos. a to W of table 1A and table 1B were melted and continuously cast to produce slabs 240 to 300mm in thickness. The resulting slab was heated to the temperatures shown in tables 2A, 2B using regenerative burners. At this time, the air in the preheating zone (preheating zone), heating zone (heating zone), and soaking zone (soaking zone) was controlled as shown in tables 2A and 2B.
The heated slab was hot-rolled at the cumulative reduction and finish rolling temperature shown in tables 2A and 2B. After hot rolling, the steel sheet was cooled at the timings and under the cooling conditions shown in tables 2A and 2B, and then wound up after cooling.
No.2 and No.8 were hot-dip galvanized.
The obtained hot-rolled steel sheets of production Nos. 1 to 38 were observed for the metal structure, and the area fraction and the average crystal grain size of each phase were determined.
The area fraction of each phase is determined by the following method.
A cross section parallel to the rolling direction of the steel sheet is etched using a nital reagent and a reagent disclosed in Japanese patent application laid-open No. 59-219473. Specifically, regarding the corrosion of the cross section, a solution obtained by dissolving 1 to 5g of picric acid in 100ml of ethanol is referred to as a solution A, a solution obtained by dissolving 1 to 25g of sodium thiosulfate and 1 to 5g of citric acid in 100ml of water is referred to as a solution B, and the ratio of the solution A to the solution B is set to 1: 1 to obtain a mixed solution, and adding nitric acid in a ratio of 1.5 to 4% relative to the total amount of the mixed solution to obtain a mixed solution as a pretreatment solution. The post-treatment solution was prepared by adding the pre-treatment solution to a 2% nital solution in an amount of 10% based on the total amount of the 2% nital solution and mixing the resulting mixture. A cross section parallel to the rolling direction of a steel sheet is immersed in the pretreatment liquid for 3 to 15 seconds, washed with alcohol and dried, and then immersed in the post-treatment liquid for 3 to 20 seconds, washed with water and dried, thereby corroding the cross section.
Then, at least 3 regions of 40 μm × 30 μm were observed at a depth of 1/4 mm from the surface of the steel sheet at a magnification of 1000 to 100000 times using a scanning electron microscope or a transmission electron microscope, and bainite, tempered martensite, ferrite, pearlite, and martensite in the metal structure were identified from the shape or the state of carbide, and the existence position of each phase and the area fraction were confirmed.
Further, the retained austenite area fraction was measured by X-ray diffraction. Specifically, first, in a cross section parallel to the rolling direction of the steel sheet at the 1/4 depth position of the sheet thickness of the steel sheet, the integrated intensities of the total 6 peaks of α (110), α (200), α (211), γ (111), γ (200), and γ (220) were obtained by using Co — K α rays, and the integrated intensities were calculated by an intensity averaging method, thereby obtaining the area fraction of retained austenite.
The average crystal grain size is determined by the following method.
The average crystal grain size was determined by visualizing crystal grains from a mapped Image, with the difference in Orientation between adjacent crystal grains being 15 ° or more, defined as one crystal grain using the EBSP-OIM (Electron Back Scatter Diffraction Pattern-organization Image Microcopy) method. When the average crystal grain size of the microstructure at a position 1/4 depth from the surface of the steel sheet in a cross section parallel to the rolling direction of the steel sheet is measured, 10 visual field measurements are performed at a magnification of 1200 times for a region of 40 μm × 30 μm, and the average of the grain sizes (effective crystal grain sizes) of crystals having a misorientation of adjacent crystal grains of 15 ° or more is set as the average crystal grain size.
The obtained hot-rolled steel sheet is pickled with a 1-10 mass% hydrochloric acid solution at a temperature of 20-95 ℃ for a pickling time of 30 seconds or longer and less than 60 seconds, and then the Ni concentration in the surface, the number density of iron-based carbides, the average depth of the internal oxide layer, and the arithmetic average roughness of the surface are determined.
The Ni concentration in the surface was determined by the following method.
The hot-rolled steel sheet as the subject was subjected to a field emission electron probe microanalyzer (FE-EPMA) using a JXA-8530F field, and the area of the measurement was 900 μm from the direction perpendicular to the surface of the steel sheet2The Ni concentration was analyzed as described above, and the Ni concentration in the measurement range was averaged. In this case, the measurement conditions were set as follows: acceleration voltage: 15kV, irradiation current: 6X 10-8A. Irradiation time: 30ms, beam diameter: 1 μm.
The number density of the iron-based carbide is determined by the following method.
A sample was collected with a section parallel to the rolling direction of the steel sheet as an observation surface, the observation surface was polished and subjected to nital etching, and 10 Field-of-view observation was performed using a Field Emission Scanning Electron Microscope (FE-SEM) at a magnification of 200000 times over a range of a sheet thickness of 1/8 to 3/8 with a depth position of 1/4 depths from the surface of the steel sheet as a center, to measure the number density of iron-based carbides.
The average depth of the internal oxide layer is determined by the following method.
A surface parallel to the rolling direction and the plate thickness direction was cut at a position 1/4 or 3/4 in the plate width direction of the pickled plate as an embedding sample, embedded in a resin sample, and then mirror-polished, and observed with an optical microscope without etching in 12 visual fields of 195. mu. m.times.240. mu.m (corresponding to 400 times magnification). When a straight line is drawn in the thickness direction, the position intersecting the surface of the steel sheet is set as the surface, the depth (position of the lower end) of the internal oxide layer in each visual field with respect to the surface is measured at 5 points for each 1 visual field, and the average value is calculated by removing the maximum value and the minimum value from the average values in each visual field, and the average value is set as the average depth of the internal oxide layer.
The standard deviation of the arithmetic mean roughness of the surface is determined by the following method.
The surface roughness of the pickled plate was measured by JIS B0601: 2013, the measurement method is obtained by measuring the arithmetic mean roughness Ra of the front and back of 12 or more samples, calculating the standard deviation of the arithmetic mean roughness Ra of each sample, and calculating the average value by excluding the maximum value and the minimum value among the standard deviations.
The steel sheets of manufactured nos. 1 to 38 thus obtained were evaluated for tensile strength, toughness (vTrs), ductility, and stretch flangeability as mechanical properties.
Tensile strength and ductility (total elongation) are measured from hot-rolled steel sheets according to JIS5, JIS Z2241: 2011 is obtained by performing a tensile test. Tensile Strength (TS) is JIS Z2241: 2011 tensile strength. The total elongation (t-EL) is represented by JIS Z2241: 2011 total elongation at break.
It is judged that preferable characteristics are obtained if the tensile strength is 980MPa or more and the ductility is 12.0% or more.
The toughness was determined by the following method. According to JIS Z2242: the transformation temperature was determined by the charpy impact test method for metal materials described in 2005.
When vTrs is-50 ℃ or lower, it is judged that preferable characteristics are obtained.
Stretch flangeability was determined by JSS Z2256: the hole expansion test method described in 2010 determines a hole expansion value, and sets the value as an index of stretch flange formability.
If the hole expansibility is 45% or more, it is judged that the preferable characteristics are obtained.
The hot-rolled steel sheet after pickling is degreased, sufficiently washed with water, and immersed in a zirconium chemical conversion treatment bath. The chemical conversion treatment bath contains (NH)4)2ZrF6: 10mM (mmol/l), 0-3 mM metal salt, pH4 (NH)3，HNO3) The bath temperature was set at 45 deg.C. The processing time was set to 120.
The hot-rolled steel sheet after the chemical conversion treatment was evaluated for chemical conversion treatability and coating adhesion.
The chemical conversion treatability was evaluated by the following method. The surface of the steel sheet after the chemical conversion treatment was observed with a Field Emission Scanning Electron Microscope (FE-SEM). Specifically, 10 visual field observations were made at a magnification of 10000 times to observe the presence or absence of an "uncovered portion" to which no chemical conversion treatment crystal was attached. At the time of observation, the acceleration voltage was 5kV, the probe diameter: observations were made at 30mm, 45 ° and 60 ° inclination angles. To impart conductivity to the sample, a 150 second tungsten coating (ESC-101, ELIONIX) was performed.
When no uncovered portion was observed in all the visual fields, the chemical conversion treatability was judged to be excellent (OK in the table).
The coating adhesion was evaluated by the following method.
After the chemical conversion treatment, the upper surface of the hot-rolled steel sheet was subjected to electrodeposition coating with a thickness of 25 μm, subjected to coating firing treatment at 170 ℃ for 20 minutes, and then cut out by a sharp-pointed knife to a length of 130mm until reaching the base metal, the electrodeposition coating was subjected to 5% salt water spraying at a temperature of 35 ℃ for 700 hours under the salt water spraying conditions shown in JIS Z2371, and then a 130mm long tape (NICIBAN 405A-24JIS Z1522) with a width of 24mm was attached to the cut-out portion in parallel to the cut-out portion, and the maximum coating film peeling width was measured when the tape was peeled off.
When the maximum film peeling width is 4.0mm or less, the film adhesion is judged to be excellent.
The results are shown in tables 3A, 3B, and 3C.
As is clear from tables 3A, 3B and 3C, production Nos. 1 to 4, 8 to 11 and 20 to 32 as examples of the present invention can secure mechanical properties required for steel sheets for automobiles even if the tensile strength is 980MPa, and can provide a chemical conversion coating film having excellent coating film adhesion with good chemical conversion properties even if chemical conversion treatment is performed using a zirconium-based chemical conversion treatment liquid.
On the other hand, in production Nos. 5 to 7, 12 to 19 and 33 to 38 in which the Ni concentration in the composition, the metal structure or the surface is out of the range of the present invention, the mechanical properties are insufficient, and the chemical conversion treatability and/or the coating film adhesion are poor. (for reference, the values in Table 3C, along with values outside the scope of the invention, are underlined for targeting attributes not reached)
Figure BDA0003019218500000321
Figure BDA0003019218500000331
Figure BDA0003019218500000341
Figure BDA0003019218500000351
Figure BDA0003019218500000361
Figure BDA0003019218500000371
TABLE 3C
Figure BDA0003019218500000381
Industrial applicability
According to the present invention, a hot-rolled steel sheet can be obtained which is an ultra-high strength steel sheet having a tensile strength of 980MPa or more and high press formability (ductility and stretch flangeability) and which, even when a zirconium-based chemical conversion treatment liquid is used, has chemical conversion treatment properties and coating adhesion equal to or more than those of a zinc phosphate chemical conversion treatment liquid. The steel sheet of the present invention has excellent corrosion resistance after coating because of excellent chemical conversion treatability and coating film adhesion. Further, the composition is excellent in ductility and stretch flangeability. Therefore, the present invention is suitable for automobile parts requiring high strength, formability, and corrosion resistance after coating.
Description of the symbols
1 base metal (Steel plate)
2 oxide layer
3 Ni enrichment part
4 zirconium based chemical conversion crystal
5 grain boundaries
6 internal oxide

Claims (12)

1. A hot-rolled steel sheet characterized by containing, in mass%, as represented by the average value of the entire sheet thickness direction:
C：0.100～0.250％、
Si：0.05～3.00％、
Mn：1.00～4.00％、
Al：0.001～2.000％、
Ni：0.02～2.00％、
Nb：0～0.300％、
Ti：0～0.300％、
Cu：0～2.00％、
Mo：0～1.000％、
V：0～0.500％、
Cr：0～2.00％、
Mg：0～0.0200％、
Ca：0～0.0200％、
REM：0～0.1000％、
B：0～0.0100％、
Bi：0～0.020％、
1 or 2 or more of Zr, Co, Zn and W: 0 to 1.000% in total,
Sn：0～0.050％、
P: less than 0.100 percent,
S: less than 0.0300%,
O: less than 0.0100%,
N: the content of the active carbon is less than 0.1000%,
the remainder contains Fe and impurities, and satisfies the following formula (1);
when the thickness is t, the metal structure at the position t/4 away from the surface contains 77.0-97.0% of bainite or tempered martensite, 0-5.0% of ferrite, 0-5.0% of pearlite, more than 3.0% of retained austenite and 0-10.0% of martensite in terms of area fraction;
in the metal structure, a metal material is selected,
an average crystal grain diameter excluding the retained austenite is 7.0 [ mu ] m or less,
the average number density of iron-based carbides with a diameter of 20nm or more is 1.0X 106Per mm2The above;
the tensile strength is more than 980MPa,
the average Ni concentration in the surface is 7.0% or more,
si + Al is more than or equal to 0.05% and less than or equal to 3.00% in the formula (1)
The elements shown in the formula (1) are mass% of the elements contained in the hot-rolled steel sheet.
2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition contains, in mass%, Ni: 0.02-0.05%.
3. The hot-rolled steel sheet according to claim 1 or 2, characterized in that an internal oxidation layer is present in the hot-rolled steel sheet, and an average depth of the internal oxidation layer is 5.0 μm or more and 20.0 μm or less from the surface of the hot-rolled steel sheet.
4. The hot-rolled steel sheet according to any one of claims 1 to 3, wherein a standard deviation of an arithmetic average roughness Ra of the surface of the hot-rolled steel sheet is 10.0 μm or more and 50.0 μm or less.
5. The hot-rolled steel sheet according to any one of claims 1 to 4, wherein the chemical composition contains, in mass%:
V：0.005～0.500％、
ti: 0.005-0.300% of 1 or 2.
6. The hot-rolled steel sheet according to any one of claims 1 to 5, wherein the chemical composition contains, in mass%:
Nb：0.005～0.300％、
Cu：0.01％～2.00％、
Mo：0.01％～1.000％、
B：0.0001～0.0100％、
cr: 0.01% to 2.00% of 1 or 2 or more.
7. The hot-rolled steel sheet according to any one of claims 1 to 6, wherein the chemical composition contains, in mass%:
Mg：0.0005～0.0200％、
Ca：0.0005～0.0200％、
REM: 0.0005-0.1000% of 1 or more than 2.
8. A method for manufacturing a hot-rolled steel sheet, characterized by comprising the steps of:
a heating step of heating a steel slab having the chemical composition according to claim 1 to 1150 ℃ or higher in a heating furnace having at least a preheating zone, a heating zone and a soaking zone and provided with a regenerative burner;
a hot rolling step of hot rolling the heated slab so that the finish rolling temperature is T2 ℃ or higher obtained by the following formula (2) and so that the cumulative reduction in the temperature range of 850 to 1100 ℃ is 90% or higher to obtain a hot-rolled steel sheet;
a primary cooling step of starting cooling within 1.5 seconds after the hot rolling step and cooling the hot-rolled steel sheet to a temperature T3 ℃ or lower represented by the following formula (3) at an average cooling rate of 50 ℃/second or higher;
a secondary cooling step of cooling from a cooling stop temperature of the primary cooling step to a coiling temperature of (T4-100) DEG C to (T4+50) DEG C at an average cooling rate of 10 ℃/sec or more, with a temperature represented by the following formula (4) being T4 ℃; and
a winding step of winding at the winding temperature,
in the heating step, the air ratio in the preheating zone is set to 1.1 to 1.9,
T2(℃)＝868-396×[C]-68.1×[Mn]+24.6×[Si]-36.1×[Ni]-24.8×[Cr]-20.7×[Cu]+250×[Al] (2)
T3(℃)＝770-270×[C]-90×[Mn]-37×[Ni]-70×[Cr]-83×[Mo] (3)
T4(℃)＝591-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo] (4)
wherein [ element symbol ] in each formula represents the content of each element in the steel slab in mass%.
9. The method of manufacturing a hot-rolled steel sheet according to claim 8, wherein an air ratio in the heating zone is set to 0.9 to 1.3 in the heating step.
10. The method of manufacturing a hot-rolled steel sheet according to claim 8 or 9, wherein in the heating step, an air ratio in the soaking zone is set to 0.9 to 1.9.
11. The manufacturing method of the hot rolled steel sheet according to claim 9 or 10, wherein an air ratio in the preheating zone is larger than an air ratio in the heating zone.
12. The method for producing the hot-rolled steel sheet according to any one of claims 8 to 11, characterized by comprising a pickling step of pickling the hot-rolled steel sheet after the coiling step using a 1-10 mass% hydrochloric acid solution at a temperature of 20-95 ℃ for a pickling time of 30 seconds or longer and less than 60 seconds.
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